Yes. The communication between nodes can be in all ways.
Normally the nodes are located in the same network, like you'd put your databases. So I'd definitely "protect" the whole network where Elasticsearch is running. Not sure I'd "protect" each machine individually but I'm not a network expert
I'd say that inside the network: open 9300 (and why not 9200). From the outside, only open 9200.
Hoe this makes sense.
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